1. Field of the Invention
This invention relates generally to a system for transferring a toned image in an electrostatographic printing apparatus. More particularly, this invention relates to an apparatus for enabling the transfer of toned images to high humidity conditioned papers.
2. Description of Related Art
Generally, electrostatographic copying is performed by exposing an image of an original document onto a substantially uniformly charged photoreceptive member. The photoreceptive member has a photoconductive layer. Exposing the charged photoreceptive member with the image discharges areas of the photoconductive layer corresponding to non-image areas of the original document while maintaining the charge in the image areas. Thus, a latent electrostatic image of the original document is created on the photoconductive layer of the photoreceptive member.
Charged developing material is subsequently deposited on the photoreceptive member. The developing material may be a liquid material or powder material. The developing material is attracted to the charged image areas on the photoconductive layer. This attraction converts the latent electrostatic image into a visible toned image. The visible toned image is then transferred from the photoreceptive member to an intermediate transfer belt and finally to a copy sheet to form a reproduction of the original document. In a final step, the photoconductive surface of the photoreceptive member is cleaned to remove any residual developing material to prepare the photoreceptive member for successive imaging cycles.
This electrostatographic copying process is well known. Analogous processes also exist in other statographic printing applications, such as, for example, ionographic printing and reproduction, where a charge is deposited on a charge retentive surface in response to electronically generated or stored images.
Typically, a corotron or other corona generating device transfers the developed toned images from the intermediate belt to a copy sheet. In corona induced transfer systems, the copy sheet is placed in direct contact with the toner image supported on the intermediate belt while a corona discharge is sprayed onto the back of the copy sheet. This corona discharge generates ions having a polarity opposite to that of the toner particles. The corona discharge causes charges and therefore electrostatic transfer fields to electrostatically attract and transfer the toner particles from the intermediate belt to the copy sheet.
Alternatively, electrostatic transfer fields and thus transfer can be induced by applying a potential difference to the substrate of a biased member, such as a bias transfer roll. The bias transfer roll contacts the copy sheet in the transfer zone and the substrate of the intermediate belt that originally supports the toner image.
Problems exist in the prior art when the transfer fields created by the transferring device cause charges to laterally conduct along the copy sheet. For example, in a high relative humidity environment, the copy sheet exhibits relatively low resistivity. The copy sheet therefore laterally conducts charge along the copy sheet. In many transfer configurations, if conductive surfaces are touching the copy sheet near the transfer zone, lateral conduction along the high humidity conditioned copy sheet can cause the potential of the copy sheet to go to the potential on these conductive members. When transferring toner from an intermediate belt to a high relative humidity conditioned copy sheet, an intermediate surface that is too conductive can be one of the conductive surfaces contacting the paper. Then, lateral conduction along the copy sheet can cause the potential of the copy sheet to go to the potential of the intermediate belt surface if the intermediate belt surface is in a critical conductivity range, especially if the contact dwell time of the paper and intermediate surface is long. The lateral conduction along the copy sheet can greatly lower or even reverse the polarity of the effective applied transfer field in the transfer zone and result in low transfer efficiency of the toner.
While the following discussion relates to a negative polarity toner system, a positive polarity system may also be similarly used with the polarity of the charges reversed as is known in the art. Low or reversal transfer fields occur, for example, in corona transfer field generation systems if the potential on the conductive surfaces contacting the high humidity conditioned paper near the transfer zone are near or more negative than the potential above the toner image on the intermediate surface prior to the transfer zone. This is because the applied charge concentration in the transfer zone flows laterally away from the transfer zone along the copy sheet. Therefore, the potential on the copy sheet in the transfer zone tends to flow to the potential of the conductors. It is mainly these potentials that determine the applied electrostatic fields in the transfer zone. Thus, when the potential on any nearby conductors is more negative than the potential above the toner image coming into the transfer zone, the transfer field reverses with high humidity conditioned papers to essentially prevent transfer efficiency of the toner image from the belt to the copy sheet. If the potential of the conductive members touching the paper is maintained to be substantially more positive (with negative polarity toners) than the potential above the toner image on the intermediate surface prior to the transfer zone, high electrostatic transfer fields can be achieved in the transfer zone to achieve greater transfer efficiency of the toner image from the belt to the copy sheet.
It is well known to provide a bias on nearby conductive baffles touching the paper to improve transfer of high humidity conditioned papers. In most cases, the applied potential on the nearby baffles is obtained by self biasing each of the baffles. Resistors, diodes, or other suitable electrical components are used to generate a voltage on the baffles due to lateral current flow along the paper from the charging sources in the transfer zone. The self bias approach allows improved transfer efficiency in most cases, but causes certain limitations due to high electrostatic fields in the pre-transfer zone prior to paper contact.
Lateral conduction of charge from the transfer field generating device can generate sufficiently high charge, and therefore electrostatic fields, in the pre-transfer zone to adversely affect the transfer of the toner. High transfer fields in the pre-transfer zone prior to intimate contact of the copy sheet to the toner image are undesirable since the high fields cause the toner to transfer across air gaps. This causes splatter of the toner past the edges of the image. High pre-transfer fields are also undesirable because they can lead to air breakdown when the Pasohen Curve is exceeded. Such air breakdown can cause toner charge polarity reversal and result in image defects and lower transfer efficiency. Pre-transfer air breakdown limits the applied transfer fields and also limits the transfer efficiency of the toner to the paper. The present invention presents certain transfer configurations with intermediate transfer systems that can prevent such undesirable conditions with high relative humidity conditioned papers.
In intermediate transfer systems, if the intermediate belt surface is too conductive, and if the contact dwell time of the high humidity conditioned paper past the region where the applied electrostatic field is generated is long, the potential difference between the intermediate belt and the copy sheet can be zero because of the lateral conduction of charge along the high humidity conditioned copy sheet to the highly conductive intermediate belt surface. This can result in nearly zero applied electrostatic transfer fields while the copy sheet is separating from the intermediate belt surface. This typically results in a lower toner transfer efficiency. The present invention presents certain intermediate belt surface resistivity conditions and certain bias transfer configurations that can prevent this from occurring.